High energy density radiation beam welding, such as is accomplished using a laser beam or an electron beam, is becoming more widely used to join one sheet to another during the course of manufacturing of product made up at least in part by the sheets. Laser welders use a highly focused beam of light energy directed onto one or both sheets to join them together, while electron beam welders direct a stream of electrons onto the sheets to be welded to heat the sheets and fuse them together.
One type of welding joint commonly used to secure one sheet to another sheet is a butt-joint where an edge portion of one sheet is placed in abutting fashion generally against an edge portion of another sheet before welding the sheets together along the region where they abut. Typically, before welding, the sheets are held in place by clamps and the beam is directed against one or both of the sheets to join them together in the weld interface region.
However, before butt-welding, accurate edge preparation of the sheets to be joined must be performed so that the edge of the one sheet is parallel to the edge of the other sheet. During edge preparation, the edge of each sheet to be joined is machined to make it parallel with the edge of the other sheets so that there is virtually no misalignment or gap between the sheets when they are abutted against each other for welding. When the sheets being butt-welded are to be later formed or shaped, such as by deep drawing, edge preparation is even more critical because the width of the weld produced using a laser is quite narrow, typically about a millimeter or less in width.
Even when done properly, edge preparation is time consuming and costly. However, if edge preparation is not properly performed, when the edges of the sheets are abutted against each other to be welded, any gap or misalignment between the sheets can result in a poor weld joint between the sheets, at least in the area of misalignment or gap. Even worse, if the gap from misalignment is too great, it results in a weld having out of specification concavities in the top bead and root side, or the sheets may not be joined together in that area, possibly causing weld failure during forming or, even more undesirably when the welded sheets are in use.
Conventional welding methods, such as shielded metal arc, submerged arc, self-shielded flux-core and gas-shielded arc welding processes, are also not suitable because the resultant welded sheets have relatively wide welds and large heat affected zones making it difficult for the sheets to be easily formed, especially deeply drawn, in the region of the weld without adversely affecting weld strength and weld integrity. This can possibly result in weld failure during forming due to cracking or peeling in the weld joint or wrinkling or buckling in the region of the weld. Conventional welding methods are also ill-suited for welding sheets having corrosion resistant or low vaporizing temperature coatings because they produce relatively wide welds destroying the coating across the region of the weld and its surrounding heat affected zone. Finally, the production rates that can be achieved using conventional welding processes are relatively slow, making their use in these types of applications economically undesirable.
Lasers have been used to butt-weld metal sheets of different thicknesses, different chemistries, different mechanical properties and different coatings before further fabricating the welded sheets, such as by forming or shaping. In order to insure a high quality weld seam that can sustain a forming or shaping operation without wrinkling, buckling, splitting or cracking, the edges to be abutted to form the weld joint must be straight and parallel within tight tolerances. For example, the maximum allowable butt-joint gap between the sheets to be welded generally specified for typical commercial applications must be less than the smaller of either 0.08 millimeters or less than 10% of the cross sectional thickness of the thinnest sheet and possess a shear to brake ratio of greater than 70% or greater than 90% when welding at speeds higher than 5 meters per minute. Additionally, these commercial welding tolerance specifications typically specify that surface mismatch can be no larger than 10% where the sheets are of the same gauge, meaning that the top surfaces of the sheets to be welded must be coplanar or substantially coplanar.
However, butt welding with sheet edges produced using traditional sheet metal processing methods such as slitting, cutting, or blanking, are not suitable for producing a butt-joint that will meet these tolerance specifications or otherwise be suitable for butt welding. This is because after metal processing, the sheet edge produced usually has one or more defects, such as burrs, undesirable roll-over or an excessive shear-to-break ratio, and also can significantly deviate in straightness along each sheet edge a producing butt joints that are unsuitable for welding and which have large and varying joint fit-up gaps along the joint. Unfortunately, the metal processing defects present can vary widely from sheet edge to sheet edge, adversely affecting welding process control as well as the repeatability of producing good quality, high strength welds. This can result in a high failure and scrap rate, particularly during forming or shaping. As such, without edge preparation before welding, the resultant butt welds produced using conventional laser welding techniques heretofore known will be of poor quality, lower strength, poor integrity and are generally unsuitable for shaping or forming after welding.
Several prior art solutions have been tried in an attempt to overcome at least some of these problems. These solutions include using special, non-standard edge and butt-joint constructions, performing special edge preparation for welding, using filler material, as well as oscillating the high energy density radiation beam across the butt-joint during welding.
Methods of butt-welding utilizing non-standard sheet edge constructions and special butt-joint arrangements are disclosed in an article in the 1985 ICALEO proceedings entitled Laser Welding of Sheet Metal Fabrications - Process Improvements and an article in the 1992 ICALEO proceedings entitled Investigation of Keyhole and Melt Pool Dynamics During Laser Butt-Welding of Sheet Steel Using a High Speed Camera. Both of these articles disclose butt-welding two sheets having guillotined edges oriented such that they form an open V-shaped butt-joint configuration. The latter article also discloses using guillotined sheet edges arranged in a closed butt-joint with the beveled guillotined edges facing downwardly away from the high energy density radiation beam. Both of these experimental welding methods and butt-joint configurations disclosed require that the edges be specially cut at an angle relative to the sheet surfaces. However, this requires more sophisticated sheet cutting equipment.
As is disclosed in the aforementioned 1992 ICALEO article, to form the V-shaped butt-joint configuration disclosed, the sheets were specially sheared to provide them with guillotined edges and oriented so that their edges form a butt-joint. In one configuration depicted in this reference, the sheets were arranged in an open "V" butt-joint configuration and angled relative to the laser beam during welding for preventing humping defects. As is further disclosed in this reference, the sheets can be angled or the beam can be angled relative to the sheets because gravitational effects are asserted as being negligible. Unfortunately, this open "V" guillotined edge butt-joint configuration can produce a weld joint of poor strength and deep concavity without the addition of additional filler material because there is a large fit-up gap at the mouth of the joint. Additionally, for sheets of particularly thin cross section, this butt-joint configuration can produce a weld of poor strength and integrity. Furthermore, this method of butt welding using this type of guillotined sheet edge and butt-joint configuration requires (2) expensive and time consuming machining to be performed to produce the desired angled sheet edge or (b) special and more sophisticated cutting, slitting or blanking equipment, making this less suitable for commercial welding applications.
Methods of butt-welding two horizontally oriented sheets are disclosed in Kamogawa et al., U.S. Pat. No. 5,245,156, and in the aforementioned 1985 ICALEO proceedings article Laser Welding of Sheet Metal Fabrications - Process Improvements. As is disclosed in both of these references, a laser beam is oscillated across the butt-joint during welding to butt-weld the sheets together. According to these references, oscillating the beam transversely across the butt-joint as it travels along the joint can allow a joint construction with larger than normal fit-up tolerances to be used. However, these methods require additional, sophisticated, and expensive controls for oscillating the laser beam.
A method of using a laser to butt-weld two sheets of differential thicknesses that are later formed is disclosed in Frings et al., U.S. Pat. No. 4,827,100. This prior art reference discloses that to practice the welding method disclosed, corrugations or joint fit-up gaps between the abutted sheet edges should not be greater than 0.04 mm and the focused spot diameter of the beam on the work piece should be no greater than 0.2 millimeters. To maintain this tight gap tolerance, special edge preparation must be performed on the sheets before welding so that the sheet edges are square, straight, and parallel, possessing a minimum of roll-over and burrs, and have a maximum shear-to-brake ratio. However, the special edge preparation required also necessitates additional and expensive manufacturing steps to be performed, is time consuming, requires expensive capital investment for the edge processing equipment, requires additional maintenance, and also wastes sheet metal material.
As such, none of these prior art welding methods can be suitably used with sheet edges that are produced using conventional metal processing methods without special and quite extensive edge preparation, filler material, or special edge configurations and butt-joint configurations.